It is generally recognized that the mouth is the beginning of the digestive tract, also known as the alimentary tract. Digestion starts as soon as you take the first bite of a meal. Chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the process of breaking it down into a form the body can absorb and use. The throat, also called the pharynx, is the next destination for food. From here, food travels to the esophagus or swallowing tube.
The esophagus is a muscular tube extending from the pharynx to the stomach. By means of a series of contractions, called peristalsis, the esophagus delivers food to the stomach. Just before the connection to the stomach there is a zone of high pressure, called the lower esophageal sphincter, which functions something like a valve to keep food from passing backwards into the esophagus. The stomach is a sac-like organ with strong muscular walls. In addition to holding the food, it is also a mixer and grinder. The stomach secretes acid and powerful enzymes that continue the process of breaking the food down. When food leaves the stomach, it is the consistency of a liquid or paste. From there the food moves to the small intestine.
The small intestine is a long tube loosely coiled in the abdomen and is made up of three segments (the duodenum, jejunum, and ileum). The small intestine continues the process of breaking down food by using digestive secretions comprising enzymes released by the pancreas and bile from the liver. Peristalsis is also at work in the small intestine, moving food through and mixing it up with digestive secretions. The duodenum is largely responsible for continuing the process of breaking down food, with the jejunum and ileum being mainly responsible for the absorption of nutrients into the bloodstream.
Three organs help the stomach and small intestine digest food: the pancreas, liver, and gallbladder. Among other functions, the pancreas secretes enzymes into the small intestine. These enzymes break down protein, fat, and carbohydrate from the food we eat. The liver has many functions, two of which are to make and secrete bile, and to cleanse and purify the blood coming from the small intestine containing the nutrients just absorbed. Bile is a compound that aids in the digestion of fat and eliminates waste products from the blood. The gallbladder is a reservoir that sits just under the liver and stores bile. Bile travels from the liver to the gallbladder through a channel called the cystic duct. During a meal, the gallbladder contracts sending bile to the small intestine. Once introduced into the small intestine, the bile and pancreatic secretions aid in the digestion of food.
Once the nutrients have been absorbed and the leftover liquid has passed through the small intestine, the remainder is passed to the large intestine, or colon. The colon is a long muscular tube that connects the small intestine to the rectum. It is made up of the ascending colon, the transverse colon, the descending colon, and the sigmoid colon that connects to the rectum. Stool, or waste left over from the digestive process, is passed through the colon by means of peristalsis, first in a liquid state and ultimately in solid form. As stool passes through the colon, the rest of the water is removed. Stool is stored in the sigmoid colon until a mass movement empties it into the rectum. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool, or feces, it empties its contents into the rectum to begin the process of elimination.
The rectum is a short chamber that connects the colon to the anus. It is the rectum's job to receive stool from the colon and to hold the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates, so that the sensation temporarily goes away.
The anus is the last part of the digestive tract. It consists of the pelvic floor muscles and the two anal sphincters (internal and external muscles). The lining of the upper anus is specialized to detect rectal contents, indicating whether the contents are liquid, gas, or solid. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The anal sphincters provide fine control of stool. The internal sphincter is always tight, except when stool enters the rectum.